A nonaqueous electrolyte secondary battery such as a lithium-ion secondary battery has a high energetic density and a characteristic allowing reduction in size and weight of an apparatus. Nonaqueous electrolyte secondary batteries are widely used for main power sources of various electronic apparatuses, power sources for industry or transportation installed in automobiles or airplanes, and main power sources or the like for domestic use.
Normally, in charging a nonaqueous electrolyte secondary battery, constant-current charge is performed, and then constant-voltage charge is performed to provide a full charge state. Increasing the charge current can reduce the charge duration, but increases the charge/discharge cycle degradation of the secondary battery to reduce the service life. While, decreasing the charge current can suppress the charge/discharge cycle degradation, but increases the charge duration. In other words, there is a trade-off relationship between the charge duration and charge/discharge cycle degradation with respect to the charge current.
Patent Literature 1 discloses the charging method of a nonaqueous electrolyte secondary battery in which constant-voltage charge is performed after constant-current charge. In this charging method, the rate of time variation in the voltage during the constant-current charge is calculated, and, when the rate of time variation changes from increase to decrease, the constant-current charge is switched to constant-voltage charge.
Patent Literature 2 discloses the charging method of a lithium-ion secondary battery in which the degradation level of the lithium-ion secondary battery is detected, and, when the degradation progresses, the set voltage is decreased and full charge is performed.